Crumbling stonework surrounding one of Canterbury Cathedral's biggest stained glass windows is to under go urgent repairs at a cost of at least £500,000.
Glass in the Great South Window will have to be removed so that unsafe stone and metalwork which holds it in place can be fixed, delaying other projects.
The window and south entrance has been fenced off to protect the public from the risk of falling masonry.
It is estimated the window repairs could take up to 12 months.
Chief executive of the Save Canterbury Cathedral appeal, Matthew Butler, said: "Studies of the window have shown that a lot of the stonework is becoming a little unsafe. There is a possibility that bits of the tracery might come off.
"Therefore for reasons of public safety as soon as we knew that we closed off the area.
"People can still look at it and still see the wonderful stained glass but they have to look at it from a little bit further back while we carry out the necessary work."
When asked what caused the problem, he said: "It could be something to do with the way the window has been patched up in the past and not really properly conserved.
"It's possible that it could be something to do with the metal bars that help hold the stained glass in place - expanding and contracting more than anyone thought.
"And when they expand and contract over hundreds of years gradually that weakens the structure."
Greater exposure to the elements on the south side of the building has also been blamed for the problem.
The 900-year-old cathedral is currently aiming to raise £50m for renovation works and recently received £460,000 for work on its library roof and windows.
The appeal reached the £9m mark in November 2008.